Effects of coaxial air on nitrogen-diluted hydrogen jet diffusion flame length and NOx emission

Turbulent nitrogen-diluted hydrogen jet diffusion flames with high velocity coaxial air flows are investigated for their NOx emission levels. This study is motivated by the DOE Turbine program\u27s goal of achieving 2 ppm NOx from gas turbine combustors running on diluted high-hydrogen fuels. In this study, effects of coaxial air velocity and momentum are varied while maintaining low overall equivalence ratios to eliminate the effects of combustion product recirculation on flame lengths, flame temperatures, and resulting NOx emission levels. The nature of flame length and NOx emission scaling relationships are found to vary, depending on whether the combined fuel and coaxial air jet is fuel-rich or fuel-lean. In the absence of differential diffusion effects, flame lengths agree well with predicted trends, and NOx emissions levels decrease with increasing coaxial air velocity, as expected. Normalizing the NOx emission index with a flame residence time reveals that a global flame strain based on the difference between the fuel and coaxial air velocities is not a viable scaling parameter, as has traditionally been used. A new scaling relationship that accounts for enhanced mixing via flame length reduction is found to provide an excellent collapse of the data with the correct Damkohler number scaling. Published by Elsevier Inc. on behalf of The Combustion Institute